1. Field of the Invention
Embodiments of the present invention relate generally to a charged particle beam writing apparatus and a charged particle beam writing method, and more specifically, relate to a method for focus adjustment for an electron beam in the electron beam writing apparatus, for example.
2. Description of Related Art
The lithography technique that advances miniaturization of semiconductor devices is extremely important as a unique process whereby patterns are formed in semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits is decreasing year by year. For forming a desired circuit pattern on such semiconductor devices, a master or “original” pattern (also called a mask or a reticle) of high accuracy is needed. Thus, the electron beam (EB) writing technique, which intrinsically has excellent resolution, is used for producing such a high-precision master pattern.
FIG. 11 is a conceptual diagram explaining operations of a variable shaping type electron beam writing or “drawing” apparatus. The variable shaping type electron beam (EB) writing apparatus operates as described below. A first aperture plate 410 has a quadrangular aperture 411 for shaping an electron beam 330. A second aperture plate 420 has a variable shape aperture 421 for shaping the electron beam 330 having passed through the aperture 411 of the first aperture plate 410 into a desired quadrangular shape. The electron beam 330 emitted from a charged particle source 430 and having passed through the aperture 411 is deflected by a deflector to pass through a part of the variable shape aperture 421 of the second aperture plate 420, and thereby to irradiate a target object or “sample” 340 placed on a stage which continuously moves in one predetermined direction (e.g., the x direction) during writing. In other words, a quadrangular shape that can pass through both the aperture 411 of the first aperture plate 410 and the variable shape aperture 421 of the second aperture plate 420 is used for pattern writing in a writing region of the target object 340 on the stage continuously moving in the x direction. This method of forming a given shape by letting beams pass through both the aperture 411 of the first aperture plate 410 and the variable shape aperture 421 of the second aperture plate 420 is referred to as a variable shaped beam (VSB) system.
In the writing apparatus, a pattern is written by deflecting a charged particle beam, such as an electron beam, by a deflector so as to irradiate a desired position, but there is a problem with use of the writing apparatus that beam drift occurs as the deflector becomes contaminated due to contaminants, etc. Therefore, correction of the beam drift is performed in the electron beam writing (refer to, e.g., Japanese Patent Application Laid-open (JP-A) No. 2007-43083). However, even if the drift amount is corrected, contaminants that cause the problem are not eliminated. Thus, in order to inhibit the beam drift, it is thought effective to remove the contaminants. Then, flowing cleaning gas in the optical column in the writing apparatus is considered to remove the contaminants.
However, a problem has been found that if the cleaning gas is flowed, the focus of an electron beam deviates.